1. Field Of the Invention
The present invention relates to anti friction linear motion bearings and, more particularly, to improvements in roller races and roller retainers for such bearings.
2. Description of the Prior Art
Linear motion bearings are well known in the art and are used extensively in a wide variety of machines, machine tools, and other equipment where one part is to be moved longitudinally with respect to another. These bearings generally include an inverted U-shaped bearing carriage mounted astride a modified I-beam or T-beam shaped rail. One or two pairs of tracks and returns are provided in the carriage for a plurality of recirculating rolling elements, such as balls or rollers. These rolling elements travel alternately through load bearing tracks and return tracks to provide movement along the rail with minimum friction. End caps are located on either end of the carriage and usually are provided with turnarounds formed therein for transferring the recirculating rolling elements from load bearing tracks to return tracks. The turnarounds are formed by a shaped track connecting a load track to a return track. End caps are provided which include an inner guide at the center of the track, to smooth the movement of the rolling elements around the curved portion from the load track to the return track and to prevent the rolling elements from bunching up in the turns. These end caps are usually formed out of plastic material using molds which form the curved tracks of the turnaround integral with the end cap. However, it is difficult to provide a mold having an inner guide integral with the end caps. In fact, the inner guides are usually molded in a separate operation and inserted into the proper position in the end cap during the assembly of the bearing. These separately molded inner guides are extremely small and easily misaligned or even overlooked during assembly. Further, where the linear motion bearings incorporate an integral lubrication system, the inner guides may be formed to be installed in only one orientation to provide adequate lubrication to the tracks. See, for example, U.S. Pat. No. 4,743,124 to Blaurock. These inner guides can be easily inverted during assembly, resulting in premature operational failure due to inadequate lubrication.
When the recirculating rolling elements are in the load bearing track area of the bearing carriage, they are held movably within the track by a rolling element retainer. This structure facilitates assembly and disassembly of the linear motion bearing by preventing the rolling elements in the load bearing track from falling out when the bearing carriage is removed from the rail. Rolling element retainers are generally of two types. In one type, half of the retainer is incorporated into each end cap and interlocks with the opposite retainer and end cap in the middle of the bearing carriage when the bearing is assembled. See U.S. Pat. Nos. 4,743,124 to Blaurock; 4,420,193 and 4,376,557 to Teramachi et al. These types of bearings are inherently difficult to mold. Further assembly is made more difficult by the need to use special equipment to load the recirculating rolling elements into the tracks.
The other type of retainer is formed as a separate structure and interlocks into the end caps for support. See for example, U.S. Pat. Nos. 4,502,737 to Osawa and 4,582,369 to Itoh. These bearings are easier to mold or fabricate than the first mentioned bearings. However, they also present difficulties in assembly. More specifically, the retainer must be held in place until the recirculating rolling elements are inserted and both end caps are secure.
Other difficulties which arise with such bearings relate to proper load distribution between the rolling elements and the races. Some bearings utilize balls or spherical rollers as rolling elements. Still others utilize straight rollers to increase the load carrying capability. See for example, U.S. Pat. No. 4,572,590 to Teramachi.
In bearings which utilize balls, the orientation of the loaded raceways, the balls and the rails are such that any applied force or moment is resisted by reaction forces in at least two raceways. While this configuration effectively distributes the load, it is limited by the contact that occurs at the reaction of the ball and either the raceway or the rail. The ability of the bearing to withstand high loads is also determined by the deformation of the body of the bearing, as such reactions may cause the ball-race/rail orientations to change.
As noted, generally the load carrying capability of linear motion bearings is increased by utilizing cylindrical rollers for the rolling elements. These bearings include a plurality of essentially cylindrical rollers in place of balls. In some instances the cylindrical rollers are chained, in order to maintain positional control. See for example, U.S. Pat. No. 4,563,045 to Katayama. While the reaction of a cylindrical roller on a flat rail or race is more effective in transmitting load, due to the increased surface area, end effects and concentrated edge loading can produce extremely high localized stresses, which can decrease the effective load which the bearing may carry. Additionally the carriage body of the bearing is subject to deformation upon loading, thus altering the orientation of the roller to the race or rail, and aggravating the high stress conditions as the roller ends.
Attempts have been made to decrease the effects of the above mentioned high stress condition in the rollers by modifying the shape of the rollers. Typical of this approach is the use of spherical rollers and lightly crowned cylindrical rollers in place of the essentially straight cylindrical rollers. The use of crowned rollers and/or spherical rollers avoids some of the edge and end loading problems described above, but inherently reduces the load capacity of the bearing, as there is a smaller contact area. Additionally, the fabrication of specially shaped spherical and crowned rollers is more difficult and expensive to produce than essentially cylindrical rollers. For example, such rollers require additional manufacturing steps such as grinding, etc.
I have invented a linear motion bearing which provides effective load distribution utilizing cylindrical rollers while incorporating a unique system of retainer elements and end caps associated therewith. The present invention thus provides improved load carrying capability for the bearing, while avoiding the above mentioned problems associated with deformation of the load carrying components. Further, the assembly of the bearing according to the present invention has been simplified, thus facilitating predictable and precise operation.